(NaturalNews) Herbicides commonly used for both agriculture and home can induce antibiotic resistance in equally common disease-causing bacteria, according to a study conducted by researchers from Mexico and New Zealand and published in the journal mBio.

"Every day you see in the news that there are concerns about the ever increasing frequency of antibiotic resistance in bacteria that can cause disease in people and our animals," researcher Jack Heinemann said in an interview with GMWatch. "Anything that contributes to this problem should be considered because new antibiotics are rare."

Previous studies have repeatedly shown that non-antibiotic chemicals (such as aspirin) can modify the degree to which bacteria tolerate antibiotics. Biocidal chemicals in particular (such as pesticides) have previously been shown to induce resistance to multiple antibiotic drugs. Yet no prior studies had looked at whether herbicides might also have such an effect.

No way to predict effects

The study was conducted on two common bacterial species, Escherichia coli and Salmonella enterica serovar Typhimurium (also known as Salmonella Typhimurium). Both are naturally found in the gastrointestinal tracts of many animals, including farm animals (E. coli also naturally occurs in the human gut), and strains of both are major causes of food-borne illness. Salmonella Typhimurium is typically found in contaminated water, milk, poultry or beef, while disease-causing E. coli typically comes from undercooked beef or contaminated raw vegetables. Modern agricultural and food-production techniques have increased the spread of these pathogens.

The researchers exposed the bacteria to three separate commercial herbicides, purchased at a local store: Kamba (active ingredient dicamba), 2,4-D (2,4-dichlorophenoxyacetic acid) and Roundup (glyphosate). The bacteria were exposed to five separate antibiotics, representing five different drug families: ampicillin (beta-lactams), chloramphenicol, ciprofloxacin (fluroquinolones), kanamycin (aminoglycosides) and tetracycline.

The researchers found that, in many cases, the antibiotic resistance of the bacteria increased dramatically, up to sixfold. In some cases, the bacteria became more vulnerable, while in others they became more resistant. There was no way to predict the direction or scale of the change ahead of time; responses varied depending on the bacteria, antibiotic and herbicide involved.

"Thus, different potential disease-causing bacteria may react differently to the same herbicide or to the same antibiotic," Heinemann said.

The researchers then sent the original bacteria and chemicals to another researcher, who carried out a blinded replication. Her results were the same.

Could drive evolution of superbugs

In the study abstract, the authors note that herbicides and other biocides are not typically tested for "sublethal effects on microbes," such as those seen in the new study. Given the growing problem of antibiotic resistance, this is a major hole in current safety testing protocols.

The herbicide concentrations used in the study were much higher than the residues found in food products but were less than the maximum application levels for home or agricultural use. Additionally, the antibiotic resistance properties of bacteria typically change when they are exposed to antibiotics and another chemical (in this case, herbicides) at the same time. Thus, the risk of herbicide-induced antibiotic resistance is greatest in agricultural settings, where both herbicides and antibiotics are in regular use. In certain cases, home use of herbicides could be a concern if a human or pet taking antibiotics were exposed to the chemicals.

The researchers note that the change in antibiotic tolerance occurs faster than the lethal effects of the drugs.

"The magnitude of the induced response may undermine antibiotic therapy and substantially increase the probability of spontaneous mutation to higher levels of resistance," the researchers wrote. "The combination of high use of both herbicides and antibiotics in proximity to farm animals and important insects, such as honeybees, might also compromise their therapeutic effects and drive greater use of antibiotics."

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